1. Field of the Invention
This invention relates to an ultraviolet-curable resin composition, and more particularly to an ultraviolet-curable resin composition which can be suitably used for the formation of a back coat layer in the two-step etching process to be used for the production of shadow masks for high fine color picture tubes. Further, the present invention relates to a method for the production of a shadow mask which uses such a composition in the formation of a back coat layer and to a shadow mask obtained therefrom.
2. Description of the Prior Art
A shadow mask having the function to collide a predetermined coloring object with electrons irradiated from an electron gun in a cathode-ray tube for a color television is a metal plate which includes a great number of minute holes prepared by etching. The formation of the minute holes is performed by preparing plenty of minute concave portions of a hemisphere or square shape having different diameters in the corresponding positions on both the front and back sides of a metal plate respectively and making the bottoms of the corresponding concave portions open for free passage.
The outline of the manufacturing process of a shadow mask is explained hereinbelow. First, a photosensitive resin coating is applied on both the front and back sides of a thin metal plate made of iron, for example. Then, a negative film having a predetermined exposure pattern is superposed on the metal plate, and the photosensitive resin coating film is subjected to exposure to light to cure the exposed areas of the photosensitive resin coating film, to a development treatment to remove the unexposed areas of the photosensitive resin coating film, and to baking thereof. Subsequently, the photosensitive resin coating film is subjected to the primary etching with a corrosive liquid such as ferric chloride to form minute concave portions on one side or on both the front and back sides so as not to be opened. Then, an ultraviolet-curable resin composition as a back-coating material is applied only on one side of the metal plate to form a film which buries the minute concave portions in one side of the metal plate and then cured by subjecting to heating and/or to irradiation of ultraviolet light, etc. After the one side of the metal plate is protected as mentioned above, the concave portions in the other side are again subjected to the secondary etching with a corrosive liquid so as to be opened for free passage in the bottom with the concave portions formed in the one side by the primary etching. Subsequently, each of the cured coating films of the photosensitive resin for pattern formation and the ultraviolet-curable resin composition as the back-coating material are removed by an alkali treatment to obtain a shadow mask.
As a composition which satisfies the performance required of the ultraviolet-curable resin composition as a back-coating material of a shadow mask, the composition obtained by dissolving in water or in an organic solvent an ultraviolet-curable resin capable of forming a film soluble in an alkaline solution is used now. This composition is applied to a substrate which has undergone the primary etching by a rolling method, a spray method and others, then transferred into a hot air drying furnace etc. to evaporate a solvent, and thereafter cured by ultraviolet-light irradiation to form a resin coating film.
The ultraviolet-curable resin composition as a back-coating material obtained by dissolving in a solvent the resin capable of forming a film, however, requires evaporation of a large amount of solvent by heating for the resin film formation. Thus, it needs a heating apparatus and a recovery system of a solvent. Moreover, there is a risk involved in terms of the toxicity accompanying the handling of the organic solvent and the possibility of a flash ignition explosion. In order to decrease any danger such as ignition, explosion, and a fire in a drying step, the use of chlorine-based solvents, which have fire retardant properties, is also under consideration. In recent years, however, these solvents have been regulated in view of problems such as air pollution. Furthermore, a coating film causes volume shrinkage with evaporation of the organic solvent. Consequently, there is a problem in that the coating film of the shoulder part (edge part) of a minute concave portion tends to become thin.
As a method for solving such problems, the method using the ultraviolet-curable resin which is essentially free of solvent may be considered. When the heretofore known alkali-soluble ultraviolet-curable resin of which cured coating film has acid resistance is used in an ultraviolet-curable resin composition as a back-coating material of a shadow mask, however, it is difficult to fill up minute concave portions uniformly with the composition, without producing air bubbles. Moreover, there is the possibility of causing such problems as poor curing properties in a concave bottom portion and the coating residues due to shortage of the alkali solubility and strippability, and poor adhesiveness to a shadow mask material. Accordingly, the composition which satisfies all the required characteristics is not yet known in the art.
Heretofore, it has been thought that such problems as described above originate in the high viscosity of the ultraviolet-curable resin composition to be used. For example, JP 1-261410,A proposes to set the viscosity of a resin composition at 25xc2x0 C. to 100 cPs or less. However, when the conventionally used ultraviolet-curable resin composition is applied to a material having a finely undulating surface by means of a roll, for example, since the resin composition has low viscosity and tends to flow, the resin composition flows out of a minute concave portion when part of the roll surface part enters into a minute concave portion and pressure is added to the coating therein, which results in shortage of filling up of minute concave portions with the composition. As a result, there is a problem that the coating film of the shoulder part (edge part) of the minute concave portion tends to become thin. Another problem is the occurrence of cissings on the surface of the metal material, i.e. a phenomenon where spot-like portions which are not covered by the coating film due to the surface tension occur. This phenomenon is also referred to as craters. Generally, when the viscosity of the composition is too low, since the cured coating film becomes uneven, it brings such problems as affording poor etching resistance or inferior alkali strippability.
In order to solve such problems, the present inventors have proposed a composition which exhibits surface tension in the range of 30 to 50 mN/m in JP 10-306124,A. The problem in the application of a composition has been solved by this alteration. However, it cannot necessarily be said that the problems of the resistance against an etching process (corrosion resistance) and the peeling of the coating film in the course of conveyance are fully solved. The problem that sufficient resistance has not been acquired is still left behind.
Accordingly, the fundamental object of the present invention is to provide an ultraviolet-curable resin composition which can be applied uniformly on the finely undulating surface of a material so as to fully fill up even inside of minute concave portions with the composition without producing air bubbles and which can be easily cured by the ultraviolet-light irradiation for a short time, without causing the problems as described above.
A further object of the present invention is to provide a non-solvent type ultraviolet-curable resin composition which is excellent in the coating workability and permits formation of a cured coating film which is excellent in adhesiveness to a metal such as a shadow mask, exhibits high etching resistance, does not exfoliate at the time of conveyance, exhibits solubility and strippability in an alkali treatment after etching, and thus can be advantageously used as a protective material for etching, particularly as a back-coating material in the production of a shadow mask.
Still another object of the present invention is to provide a method for the production of a shadow mask, which method is capable of relatively easily and effectively forming a back coat layer excellent in etching resistance and solubility and strippability in an alkali treatment after etching by bringing the characteristics of the ultraviolet-curable resin composition as mentioned above into play to the maximum extent and forming a great number of perforations with high precision, thereby providing a highly fine and high quality shadow mask.
To accomplish the objects described above, the present invention provides an ultraviolet-curable resin composition characterized by comprising (a) a compound having one or more acryloyl or methacryloyl groups in its molecule and a phosphorus atom, (b) a compound having at least one carboxyl group and one acryloyl or methacryloyl group in its molecule, and (c) a compound having one or more acryloyl or methacryloyl groups in its molecule, wherein the compound (a) mentioned above is incorporated therein in an amount of 0.5 to 10 parts by weight, based on 100 parts by weight of the total amount of the compounds (b) and (c) mentioned above, which exhibits the surface tension at 25xc2x0 C. in the range of 30 to 50 mN/m.
A more specific mode of the present invention provides an ultraviolet-curable resin composition characterized by further comprising (d) a leveling agent and (e) a photoinitiator besides the components (a) to (c) mentioned above.
The ultraviolet-curable resin composition as mentioned above is excellent in the coating workability and its cured coating film has the characteristics of exhibiting high etching resistance and outstanding solubility and strippability in an alkali treatment after etching. Therefore, this composition can be particularly advantageously used as an ultraviolet-curable resin composition for a back-coating material in the production of a shadow mask.
Accordingly, in accordance with another aspect of the present invention, there is provided a back-coating material to be used for the production of a shadow mask, which material consists of the aforementioned ultraviolet-curable resin composition. Preferably, the back-coating material which exhibits the contact angle of 25xc2x0 or less to a substrate of a shadow mask is used.
Furthermore, the present invention also provides a method for the production of a shadow mask which uses the aforementioned ultraviolet-curable resin composition in the formation of a back coat layer in the manufacturing process of a shadow mask and a shadow mask obtained therefrom. The method of the present invention is a manufacturing method of a shadow mask comprising at least a step of forming a photosensitive film on both sides of a shadow mask substrate consisting of a strip-like thin metal plate, a step of subjecting both sides having the photosensitive film formed thereon to exposure to light according to a predetermined pattern and to development, thereby exposing part of the metal surface according to the predetermined pattern to form an etching resist, a primary etching step for etching at least one surface of the thin metal plate having this etching resist formed thereon and the partially exposed metal surface, thereby forming concave portions in the exposed part of the metal surface of at least this one surface, a step of forming a back coat layer by applying a back-coating material on one surface having the concave portions formed in the primary etching step and filling up these concave portions with the back-coating material, a secondary etching step for etching a surface opposite to the surface having the back coat layer formed thereon to form concave portions which communicate with the concave portions formed in the primary etching step, and a stripping step for removing the back coat layer and the etching resist, characterized in that the aforementioned back-coating material is used and cured by ultraviolet-light irradiation in the step of forming the back coat layer mentioned above.